For purposes of possible transplantation to a recipient eye, the cornea from a donor eye is typically excised as a generally circular section of tissue that includes a small outer rim of scleral tissue.
This corneal tissue is then generally subjected to a variety of physical manipulations and procedures, including storage, transport, microscopic evaluation and lastly, "trephining" a procedure in which a circular plug, i.e., "donor button", of the desired size is cut out from the desired location for use as an transplant.
An excision and storage technique commonly used in many eye banks is outlined in the chapter entitled "Tissue Processing", by M. A. Gallagher, pp. XI-1 through XI-4, in Eye Bank Technician Manual, Eye Bank Association of America, Houston, Tex., 1984. In this technique the corneal tissue is cut out, i.e., excised with small scissors in a generally circular shape having a 2-3 millimeter outer rim of scleral tissue. The tissue is typically transferred to a clear vial, and stored, free-floating, in sterile storage medium. Certain recent developments in the formulation of storage media purport to enable the storage of such explants for up to several days or even weeks, see, e.g., B. F. Boyd, Highlights of Ophthamology Letter, Vol. XIV(2): 1-16 (1986).
The corneal tissue is typically stored in the free-floating state until just before use when it is microscopically examined to evaluate the integrity of its endothelial surface, e.g., by removing the tissue with a forceps to a viewing chamber or viewing it directly in its storage vial as described, e.g., in "Wide Field Specular Microscopy of Excised Donor Corneas", C. W. Roberts et al, Arch. Ophthalmol. 99:881.varies.883 (1981). The condition and appearance of the endothelial and epithelial surfaces are critical factors to be determined in deciding whether a particular corneal tissue is suitable for implantation. Abrasions and loss of cells from the endothelial surface are major factors for rejecting many corneal tissues for use and can be attributed, at least in part, to damage done to the tissue during and by virtue of its storage.
In an adaptation of the free-floating storage system, Coopervision, Inc., Irvine, Calif. has a commercially available storage jar called a "PRO CSVC" which is described as a "Corneal Storage and Viewing Chamber" for use in conjunction with its microscope systems. The chamber consists essentially of two parts, i.e., a translucent plastic jar and a clear plastic cap.
The distinguishing feature of the jar is a circle formed by eight plastic spikes extending upwardly from the base of the jar, each being inwardly notched and then tapered downwardly at their tips so as to provide a support upon which corneal tissue can rest without falling between the spikes or into the open circle defined by their center.
The cap has a circular indented portion that is optically clear and configured to allow a microscope lens to penetrate the plane of the top of the cap a distance almost equal to the height of the cap, and to move about therein, in order to scan and focus on the corneal tissue when the tissue rests in the center of the chamber.
The cap indentation also serves to provide a barrier at the top of the jar chamber, thereby restricting the ability of the tissue to float out of its chamber. The tissue is nonetheless still free to move within the confines of the chamber, such that the jar might need to be tapped or swirled in an attempt to bring the tissue to rest in a centered position at the base of the chamber.
As with the free-floating storage vial described earlier, the tissue would of course, need to be physically grasped and removed from the chamber in order to place it carefully in a trephining device.
The basic trephining devices are simply cutting blocks and corneal punches. The tissue is carefully placed epithelial side down in a concave indentation in a block made out of a hard inert material such as Teflon, such that the center of the tissue is aligned with the center of the indentation, and the tissue rests in approximately its normal curvature during trephining. A circular metal trephine blade, attached to a punch mechanism, is then carefully aligned and oriented, in a manner analogous to a drill press, so as to hover above or lightly touch the tissue at the desired, generally central, location. The blade is then tapped or turned down into the tissue with sufficient force and to a sufficient distance to cut out a plug.
Devices have been described for securely holding corneal tissues during trephining, see e.g., U.S. Pat. Nos. 2,929,603, 3,058,471 and 4,077,411. U.S. Pat. No. 4,077,411 for instance, describes an apparatus having a spring-loaded ring to secure the corneal tissue at its edges over a semi-spherical post. The introduction of a harmless liquid from below the tissue, through a conduit in the post, is said to provide a cushion to resist the downward pressure of the trephine blade.
In "Corneal Holder", Amer. J. Ophthalmol. 80(3) Part II:551-552 (1975), there is described a holder having a semi-spherical pedestal, a matching scleral sealing sleeve and a retaining ring. The sleeve retains the tissue on the pedestal at its scleral rim, and the ring locks the sleeve to the pedestal in a manner said to trap a cushion of air beneath the tissue, again to resist the downward pressure of the trephine blade.
It follows that throughout the typical functions involved from excision to implantation, the corneal tissue must frequently be handled a variety of times, e.g., by forceps, in order to place it in the storage vial upon excision, remove it and prepare it for microscopic evaluation, and place it and then orient it in a device or on a block for trephining. Each physical manipulation increases the chance of damage to the tissue, particularly at its edges and on its endothelial surface, and requires the patience, time and skill of a trained technician.
Throughout these functions, it would be highly desirable to be able to orient the corneal button with respect to its placement in the recipient eye. Kiely, et al, "Meridional Variations of Corneal Shape", Amer. J. Optom. Physiol. Optics, 61(10):619-626, 1984, for example, explains that the cornea in fact consists of four individual corneal meridians, each with its own radius of curvature. If the corneal tissue is transplanted into the recipient eye in a manner in which its radii are incompatible with those of the recipient eye, astigmatism can result. In order to limit this situation, surgeons currently must generally transplant corneal buttons in an unknown orientation in the eye, and then tighten and loosen the stitches holding the transplant, in an effort to hold the transplant in its desired configuration.
Currently practiced corneal tissue handling procedures do not generally allow the orientation of the corneal tissue or button. In fact, orientation is essentially lost from the moment the tissue is excised and placed to float freely in its storage medium.
Furthermore, it has been found that it may be desirable in some situations to prevent or lessen fluid absorption into the cut edges of the corneal tissue during storage. Undue absorption can lead to a swelling of the explant, thereby thickening it beyond use. One approach currently used in an attempt to avoid such thickening has been the development of modified storage media, as described in "Minnesota System Corneal Preservation" Lindstrom, et al, Brit. J. Ophthalmol., 70:47-54, 1986.